The Muller and Mauthner neurons of the lamprey have morphological characteristics such that they can be identified and studied individually. By means of experimental histological methods at both light and electron microscopic levels, the afferent and efferent connections will be exactly determined in animals of various ages from the ammocoetes form through metamorphosis to the adult. Correlated with the study of connections will be observations of the gross behavioral and anatomical changes. Because these neutrons are likely responsible for simple reflect patterns, the results should enable the definition of the morphological substrate for the establishment of the behavior and for the investigation of its modification in terms of neuronal events.